Dynamics of pile-supported structures in seismically liqueable soils

Failure of pile-supported structures are still observed in liqueable soils after most major earthquakes. As a result, the behaviour of pile foundations during liquefaction phenomena remains a constant source of attention to the earthquake engineering community. In this context, the present research attempts to investigate the eects of soil liquefaction on the dynamic behaviour of pile-supported structures. Firstly, the thesis reports a eld investigation carried out in the region aected by liquefaction events observed after the 2012 Northern Italy earthquake sequence. The collected information is used to gain insight into the mechanism governing the onset of liquefaction in the real eld. The thesis also presents a series of multi-stage cyclic triaxial tests that aim to investigate both pre and post-liquefaction behaviours of two types of silica sands. These ndings are subsequently used to develop a new set of p 􀀀 y curves which are capable of capturing the strain-hardening behaviour observed in the experiments. The main component of the present research consists of an experimental investigation carried out using a shaking table. A preliminary
study is reported in which tests are performed to investigate the eects of artificial boundaries of the model container on the response of the enclosed soil. It is found that the use of absorbing boundaries, which are made of conventional foams, is capable of reducing reflections and generation of body waves from the articial boundaries, which in turn minimise the so called boundary eects. Subsequently, four physical models consisting of two single piles and two 22 pile groups are tested on a shaking table. The dynamic response is conveniently identied in terms of two modal parameters, namely: fundamental period and damping ratio. The experimental results suggest that the fundamental period of the pile-supported structures may increase considerably due to the soil softening caused by liquefaction. On the other hand, the damping ratio of these structures increases to values in excess of 20%. Based on the these ndings, it is noted that the seismic demand imposed by the shaking on the models may vary with the excess pore water pressure generation, which in extreme case may lead to full liquefaction conditions. In particular, it is observed that the highest acceleration demand, and consequently maximum inertia force, experienced by the models occur during the transient to liquefaction. Finally, a series of numerical analyses are performed using the Winkler approach with the
proposed p-y curves. The numerical results show that the models correctly replicated the distribution of the maximum bending moments measured after the onset of liquefaction but consistently underestimated the maximum moments by a factor ranging from 2 to 3. It is also found that the capacity spectrum method can be used as a simple and convenient tool for assessing the seismic demand.